Asphalt has been primary material for water proofing and bonding in pavement and construction industries. Higher performance demands have resulted in modifying the asphalt in various ways to improve its quality. Of importance is the need for asphalt's ability to withstand higher temperatures without softening. The normal range at which most asphalt grades soften is 45 to 50° C. In order to increase the softening point, the asphalt is modified by the addition of various polymers ranging from thermoplastic resins, such as polyethylene, polypropylene to thermoplastic elastomers like Styrene Butadiene Styrene, and Rubbers-Styrene Butadiene Rubber, Natural Rubber, Polybutadiene and so on. At the same time addition of these polymers should not affect the low temperature flexibility of the asphalt polymer composition to avoid freeze/thaw fatigue cracking.
Major challenges in polymer modification of asphalt are miscibility of the polymer with asphalt and its stability which is defined as resistance to phase separation during storage and transportation at elevated temperatures. An additional requirement is that the viscosity of polymer modified asphalt should be low enough to flow and coat the surface of aggregate in pavement construction.
Above all these, is the need to keep the cost of modification sufficiently low.
In order to achieve the economics there had been efforts since 1960s to introduce rubber recovered from used tires and other post consumer scrap rubbers for asphalt modification. The move to use tire rubber stems from need to address the used tire disposal problems as well.
Therefore, there exists a need to develop a recycled used tire and other post consumer rubber scraps modified asphalt that matches the conventional virgin rubber modified asphalts in its quality and cost.
In the past there had been use of recycled polyethylene in a process for the manufacture of recycled polyethylene plastic modified asphalt for use as binder in pavement application. The resulting products using the said Polymer Modified Asphalt improved softening point of asphalt—a desirable property for pavement binder asphalt—and improved resistance to rutting in pavements. However, the polyethylene in the modified asphalt had a tendency to float to the surface on storage and transportation.
Similarly, other common polymer modified asphalt binders that use Styrene Butadiene Styrene (SBS) copolymers as the preferred polymer over many others had the same tendency of phase separation as exhibited by the lighter SBS phase rising to the surface.
Hence, compatibility of the polymer with asphalt is essential to ensure that the properties of the mix are maintained in the finished product to achieve desired storage stability and performance criteria. Phase separation leads to the polymer modified asphalt unsuitable for use in paving binder, roofing applications, and other specific products. In the past, various methods for improving storage stability have been suggested.
One method requires continuous mixing of the asphalt/polymer mixture to avoid phase separation (See, for example, U.S. Pat. Nos. 4,240,946 and 4,314,921) comprises mixing and homogenizing molten bitumen and a polymer in a hot state between 260° C. and 310° C. for several hours. Polymers, especially natural rubber present in tire rubber begin to degrade above 180° C. Hence this is not the solution for recycled tire rubbers that is mostly composed of Natural Rubber, and consequently the performance of the modified asphalt would be impaired due to degraded rubber in the final mix.
In other methods for storage stable asphalt compositions, U.S. Pat. Nos. 5,336,705, 5,627,225, and 5,348,994, disclose a process that uses inorganic acids to stabilize and then neutralize with alkalis in the preparation of polymer modified asphalt compositions. The disadvantages of the processes are high cost and corrosion tendency of the acids on the containers.
In yet another method disclosed in U.S. Pat. No. 5,070,123, it relates to use of an inorganic acid to form an acid treated asphalt mixture before adding modifying polymer to produce storage stable polymer modified asphalt composition. This too has the same disadvantages as the inorganic acid modification described in U.S. Pat. No. 5,336,705 and others above.
However, none of the above methods by themselves or in combination disclose a composition nor the process for storage stabilized devulcanized recycled tire rubber modified asphalt using derivatives of Natural Rubber and synthetic copolymer. Early work in use of ground tire rubber (GTR) in asphalt binders was carried out by Charles McDonald, an engineer with Arizona Department of Transport in 1964. He was issued with patent on use of GTR in pavement mixes around 1970. Since then Arizona had been in the fore front of GTR modified asphalt rubber in binders for pavement application. Other states such as Florida, Virginia, Mississippi and California followed the example. However problems related to preparing a homogenous asphalt/GTR mix resulted in many discontinuing the use of GTR modified asphalt widely. The most serious issues were the ability to dissolving the GTR in asphalt and keeping the GTR suspended without settling to the bottom. GTR manufacturers resorted to producing finer ground powders to overcome these issues, but at higher GTR cost. Yet the problem of dissolving even the finer GTR and keeping it stable without separation during storage at elevated temperatures remain to this day.
As such it has always been desirable to find a solution to making the GTR or its derivatives soluble in asphalt and keeping it stable in suspension during storage before use. There were a number of methods developed to achieve solubility and avoid phase separation, one of which is U.S. Pat. No. 5,095,055 teaches use of branched polymer to an asphalt that has been previously treated with an organic acid. Yet another (U.S. Pat. No. 5,070,123) describes a method of treating the asphalt with inorganic acid prior to addition of modifying polymer to improve the storage stability of the composition. In both these cases the polymers used for modification were, unlike the present invention, virgin polymers and not recycled ground tire rubber.
Yet a more recent patent (U.S. Pat. No. 7,968,628 B2) teaches a process for producing an asphalt and crumb rubber composition that has one or more acids and a cross linking agent for use as pavement binder. It does not mention if the composition is storage stable nor if the crumb is miscible in asphalt so that there is no phase separation.
Hence, this invention relates the solution to ground tire rubber (GTR) derived recycled and devulcanized tire rubber miscibility in asphalt and its storage stability as measured by “cigar tube” separation test which illustrates the phase separation between top and bottom layers of an asphalt/polymer composition through difference in softening point and rheology. The GTR derived recycled and devulcanized tire rubber used in this invention is manufactured and marketed by Quantum Polymer Composites, LLC of Cleveland, Ohio. The product sold under the brand name of “Ecorphalt” can be dissolved in asphalt with the aid of compatibilizers/stabilizers that will also keep the composition stable without separation on storage at elevated temperatures at the terminal and during transportation.